Choline, also known as choline base, is a colorless liquid and a strong organic base. Chemically, it is trimethyl(2-hydroxyethyl)ammonium hydroxide and can be represented by the formula [(CH.sub.3).sub.3 NCH.sub.2 CH.sub.2 OH].sup.+ OH.sup.-. It is not usually encountered as the free base but as a salt; the one most commonly used is chloride.
Choline chloride is an important nutritional substance, is used clinically in liver disorders and as a constituent in animal feeds. Choline base is especially suitable for use as a photoresist stripper. Aqueous and C.sub.1 -C.sub.8 alcoholic solutions of choline are useful in electronic applications such as photoresist developing and stripping, anisotropic etching and cleaning silicon wafers, furnace tubes and the like.
Among the choline derivatives which have been or are being produced on a commercial scale, there may be mentioned choline hydroxide, choline chloride, choline bitartrate, choline dihydrogen citrate, choline bicarbonate, choline bisulfate, tricholine phosphate and tricholine borate. These derivatives and other unnamed derivatives as well as choline-like compounds in basic solutions can be stabilized in accordance with the present invention.
Choline base itself is ordinarily sold as an aqueous or methanolic solution which is nearly colorless when received from the manufacturer. There is a tendency to yellow, i.e., to decompose, particularly during shipment and storage. This tendency has the unwanted consequence of darkening and forming precipitates which impair the efficacy of the base as a developing or stripping agent. The color can be removed by treatment with activated carbon but this causes added expense and inconvenience to the user. Furthermore, choline treated in such fashion soon begins to form color again. To prevent or retard such decomposition and resulting discoloration with its consequences, it would be desirable to provide an inexpensive stabilizer for choline base that would not interfere with the intended use in any way and retain its water-white color over long periods of time.